This invention relates generally to high efficiency power amplifiers for radio frequency applications and more specifically to a solid state power amplifier circuit which realizes high efficiency conversion of supply power to radio frequency signal power by operating in a switched mode with control of fundamental and harmonic frequency impedance in the output circuit.
Traditional power amplification at radio frequencies has been achieved with thermionic and solid state devices operating in the range of their linear amplification characteristic or, where possible, in a combination of linear characteristic and cut-off characteristic. These operating ranges, while extremely useful, are not the most efficient at producing radio frequency (RF) output from applied signal and direct current (DC) supply power. A number of investigators have researched techniques for improving the input power to output power conversion efficiency and have concluded that optimum efficiency could be realized by operating the active device as a switch. This class of operation would cause the device to be in a saturated or cut-off condition most of the time and therefore dissipate as little power in an active region as possible.
Practical realization of high frequency switching operation, however, has been difficult to achieve due to finite switching times in the active device and device and package parasitic impedances. Increasing use of portable, battery powered, radio transmitters creates a significant demand that the efficiency of the power amplifier stage become as great as possible.